Semiconductor laser assembly

ABSTRACT

A semiconductor laser assembly is provided with a photodetector module and a laser module mounted on orthogonal surfaces of a heatsink. The photodetector module includes a photodetector and two electrically isolated wire bond blocks, each with two surfaces parallel to the orthogonal surfaces of the heatsink. The laser module includes a laser diode bonded and a wire bond region with two surfaces also parallel to the orthogonal surfaces of the heatsink. Both of the wire bond block surfaces of the photodetector modules are thus parallel to the wire bond region surfaces on the laser module, allowing wire connections to be made between parallel surfaces before the heat sink is mounted on a header. One of the surfaces of each of the wire bond blocks and regions is parallel to the pins of a header, allowing attachment of wires from the photodetector module and the laser module to the pins without rotating the header and pins.

FIELD OF THE INVENTION

This present invention relates to a semiconductor laser assemblycontaining a laser diode for emitting a laser beam and photodetectorsfor detecting the laser beam reflected from an external target.

BACKGROUND OF THE INVENTION

A laser diode is a semiconductor device which emits light ofsubstantially a single wavelength. The light from a laser diode can befocused to a spot with a diameter comparable to the light wavelength.The laser diode belongs to the same family of semiconductor devices asthe LED (light emitting diode). However, the light from an LED has abroader spectrum of wavelengths and thus cannot be focused as sharply asa laser diode. The configuration and composition of the laser diodedetermines its wavelength, expected lifetime and light guidingmechanism.

Semiconductor laser diodes are generally mounted on a substrate orstructure which provides the electrical, thermal, and spatial needs ofthe laser diode for the intended application. The combination of asubstrate or structure with a laser diode is commonly referred to as alaser assembly. Examples of laser assemblies are shown in U.S. Pat. Nos.3,257,626, 3,293,513, 3,457,468, 3,479,613, and 4,483,480.

Most commercial laser assemblies contain a photodetector behind thelaser to monitor the level of light emitted by the laser diode. Themonitoring photodetector provides a signal which is used to maintain aconstant laser output. It has been noted in some applications that whena small amount of light emitted from the laser is fed back into thelaser diode, it causes a change in the laser output power which can bedetected by the monitoring photodetector.

Many commercial applications use a second photodetector in front of andbelow the laser diode to monitor laser light reflected off of an objectlocated in front of the laser diode. The second photodetector provides asignal which can be used to determine the reflective properties of theilluminated portion of the object such as optical storage media.

FIGS. 1a and 1b show front and side views, respectively, of a typicalprior art laser assembly 10. A semiconductor laser diode 12 and a tiltedrear photodetector 13 are mounted on a metal heatsink 14 which, in turn,is mounted on a header 15. The header has three output pins 16, 18, and20 for external connections. The photodetector is slightly tilted and islocated behind the laser diode as shown in the side view of FIG. 1b.This tilt of the photodetector is provided to prevent laser light, whichis reflected off of the photodetector, from being fed back into thelaser diode.

A wire 22 connects the top or anode of the laser diode to a flat surfaceon pin 16 that is parallel to the top of the laser diode. A wire 24connects the front or anode of the photodetector to a beveled end of pin18. The bottom or cathode of the laser diode is electrically coupled topin 20 through the heatsink and the header 15. The cathode of thephotodetector is also electrically coupled to pin 20 through the header.This provides a three pin laser assembly which is typically packaged ina standard T05 can. Because the cathode of the laser diode shares pin 20with the cathode of the photodetector, the laser assembly is sometimescalled a common cathode device.

Because wires must be bonded on a horizontal surface due togravitational effects, the laser assembly, including the header andpins, must be placed on its side as shown in FIG. 1b during bonding ofwire 22 from the anode of the laser diode to pin 16. After wire 22 isbonded, the laser assembly is then rotated approximately 90 degrees tothe orientation shown in FIG. 1(a) for bonding wire 24 from the anode ofthe rear detector to pin 18. As a result, the bonding process is mademore complex because the header and pins must be rotated.

In an alternative assembly, the laser diode is attached to the heatsinkwith the anode of the laser diode in contact with the heatsink and thecathode of the laser diode in contact with wire 22. In this alternativeassembly, the cathode of the photodetector shares pin 20 with the anodeof the laser diode. A variation to the three pin devices is a four pindevice which uses a silicon submount to electrically isolate the cathodeof the laser diode from the heatsink.

In any of the above-described prior art assemblies, the header and pinsmust be rotated during the bonding process.

FIG. 2a shows the electrical circuit of the common cathode laserassembly shown in FIGS. 1a and 1b. Corresponding elements to FIGS. 1aand 1b have primed corresponding numbers. The anode of laser diode 12'is electrically coupled to pin 16'. The anode of photodetector 13' iselectrically coupled to pin 18'. The cathodes of laser diode 12' andphotodetector 13' share pin 20'.

FIG. 2b shows the electrical circuit of the alternative to the laserassembly shown in FIGS. 1a and 1b. Corresponding elements to FIGS. 1aand 1b have primed corresponding numbers. The cathode of laser diode 12'is electrically coupled to pin 16'. The anode of photodetector 13' iselectrically coupled to pin 18'. The anode of the laser diode and thecathode of the photodetector are electrically coupled to pin 20'.

FIG. 3 shows a prior art laser assembly 30 as disclosed in JapanesePatent Application Number 59-117279(A). This laser assembly uses rearfacet detection of the laser output to maintain a constant laser output.A laser diode 32 is attached to a silicon submount 34 which, in turn, ismounted on a copper heatsink 36 with a plurality of output pins 38. Thesilicon submount is also a photodetector for monitoring the rear facetemission of the laser diode.

The upper surface of the silicon submount next to the heatsink isgenerally the cathode of the photodetector and is electrically coupledto the copper heatsink by physical contact. A first pole of the laserdiode is generally connected to the cathode of the photodetector 34 by awire. The anode of the photodetector and a second pole of the laserdiode are electrically coupled to the pins by separate wires.

Given the geometry of the laser assembly, the wires are first attachedto the laser diode and the photodetector, the laser assembly is rotated90 degrees, and then the wires are attached to the pins. Again, theheader and pins are rotated during the bonding process.

FIG. 4 shows a front view of a laser assembly 40 as disclosed in U.S.Pat. No. 4,757,197. A semiconductor laser diode 42, a tilted rearphotodetector 44 (similar to tilted rear photodetector 13 of FIGS. 1aand 1b), and a forward four quadrant photodetector 46 are mounted on aheatsink 48. The heatsink, in turn, is mounted on a header 50 which hasa plurality of output pins 52a-h. The tilted rear photodetector is usedto detect rear facet emissions of the laser diode in order to maintain aconstant laser output. The forward four quadrant photodetector detectsthe laser light from the laser diode that has reflected off of an objectlocated in front of the laser diode.

A wire 54 electronically couples the top, or anode, of semiconductorlaser 42 to pin 52a. Four wires 56a-d connect a first pole (preferablythe anode) of the four quadrants of the four quadrant photodetector topins 52b, 52d, 52e, and 52f, respectively. Wire 58 connects a commonsecond pole (preferably the cathode) of the four quadrant photodetectorto pin 52c. Wire 60 connects the anode of the rear photodetector to pin52h. The cathode of the laser diode and the rear photodetector areelectrically coupled to pin 52g through the heatsink and wire 62.

The laser assembly must be placed on its side while bonding wire 54 fromthe anode of the laser diode to pin 52a and wire 62 from the cathode ofthe laser diode to pin 52g. In addition, wires 56a-d and 58 are bondedfrom the poles of the four quadrant photodetector to the output pins.Furthermore, wire 60 is bonded from the anode of the rear photodetectorto pin 52h. As with other prior art, the laser assembly, including theheader and pins, must be rotated during the bonding process.

The prior art laser assembly 10 shown in FIG. 1 has many applications,one of which is in the optical head assembly 70 shown in FIG. 5. Thelaser assembly contains a laser diode to provide a light beam 72 which,after passing through a grating 74, a beam-splitter 76 and a collimatinglens 78, is focused on an information medium 80 at spot 81 by anobjective lens 82. The objective lens is supported by a focus andtracing actuator 83. When the light beam is reflected off theinformation medium, part of the beam is reflected by the beam splitterthrough a cylindrical lens 83 to a photodetector 84. The signal recordedon the information medium can be read out from photodetector 84.

FIG. 6 shows an optical head assembly 85 using the prior art laser anddetector assembly 40 shown in FIG. 4. The laser and detector assemblyradiates a laser beam 86 to a collimating lens 87. The collimated beampasses through a hologram lens 88 to an objective lens 90 and focus onan information medium 91 at spot 92. The objective lens is supported bya focus and tracking actuator 93. When the light beam is reflected offthe information medium, part of the beam is diffracted by the hologramlens and focuses on the detector part of the laser and detectorassembly. A recorded signal on the information medium can be recoveredfrom the signal coming out from the detector.

SUMMARY OF THE INVENTION

An improved semiconductor laser assembly is provided with aphotodetector module and a laser module mounted on orthogonal surfacesof a heatsink. The photodetector module includes a photodetector and twoelectrically isolated wire bond blocks, each with two surfaces parallelto the orthogonal surfaces of the heatsink. The laser module includes alaser diode and a wire bond region with two surfaces also parallel tothe orthogonal surfaces of the heatsink. Both of the wire bond blocksurfaces of the photodetector modules are thus parallel to the wire bondregion surfaces on the laser module, allowing wire connections to bemade between parallel surfaces before the heat sink is mounted on aheader. One of the surfaces of each of the wire bond blocks and regionis parallel to the pins of a header, allowing attachment of wires fromthe photodetector module and the laser module to the pins withoutrotating the assembly.

The wire bond blocks of the photodetector module are preferably deepdiffusions in a semiconductor chip containing a photodetector. A sawcuts the chip after it is attached to the heat sink to isolate the wirebond blocks from the photodetector. The wire bond region of the lasermodule is preferably a deep diffusion in a semiconductor chip to whichthe laser diode is attached. The laser diode is preferably attached sothat an electrical connection exists between the anode and both surfacesof the wire bond region.

In a preferred embodiment, the laser module is provided with aphotodetector located behind the laser diode. The photodetector providesrear facet laser detection for controlling the emissions of the laserdiode. The laser module can be further provided with a reverse surgecurrent diode to prevent damage to the laser diode from reverse surgecurrent. In the preferred embodiment, the reverse surge current diode isa photodetector diode that has been covered with a metal layer toprevent light detection and to provide a wire bond surface to one of theelectrodes of the diode. A metal covered photosensitive diode is used sothat only a single diffusion step is needed to form both the rear facetphotodetector and the reverse surge current diode.

In a preferred method, the photodetector module is fabricated andattached to the heatsink. The laser diode is then bonded to the lasermodule which is bonded to the heatsink. Wires are then bonded betweenelements on the side surface of the laser module, and from the sidesurface on the laser module to the side surface on the photodetectormodule. The assembly is rotated 90° and the heatsink is bonded to theheader. Wires are then bonded from the pins to either the top surface ofthe photodetector module or to the top surface of the laser module.

As a result of the structure of the elements of the laser assembly andof the process of assembly, the header and pins do not need to berotated during the bonding process. This greatly simplifies the bondingprocess and allows current commercially available bonding equipment tobe used without requiring intervening manual steps that are timeconsuming and prone to error.

The saw cut method of fabricating the photodetector module requires fewsteps and automatically aligns the wire bond block surfaces with thesurfaces of the heatsink.

The laser assembly may be used in many applications. However, apreferred application would be in the optical head assemnblies shown inFIGS. 5 and 6.

For a further understanding of the nature and advantages of theinvention, reference should be had to the ensuing detail descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b are front and side views, respectively, of a typicalprior art laser assembly;

FIG. 2a is a circuit diagram of the common cathode laser assembly shownin FIGS. 1a and 1b;

FIG. 2b is a circuit diagram of an alternative embodiment to the laserassembly shown in FIGS. 1a and 1b;

FIG. 3 is a side view of a prior art laser assembly using rear facetdetection of the laser output to maintain a constant laser output;

FIG. 4 is a front view of a hybrid laser assembly disclosed in U.S. Pat.No. 4,757,197 containing a laser diode and multiple photodetectors;

FIG. 5 is a top view of a prior art optical head assembly using thelaser assembly of FIG. 1;

FIG. 6 is a top view of a prior art optical head assembly using thelaser detector assembly of FIG. 4;

FIGS. 7a and 7b are front and side views, respectively, of a preferredsemiconductor laser module for generating a laser beam;

FIGS. 8a and 8b are side and top views, respectively, of a preferredphotodetector module on a heatsink for detecting reflected laser light;

FIG. 9a is a side view of the photodetector module and heatsink of FIGS.8a and 8b after the laser module has been mounted on the heatsinksurface and the appropriate wire connections are made;

FIG. 9b is a circuit diagram of the device shown in FIG. 9a; and

FIGS. 10a and 10b are top and side views, respectively, of a preferredlaser assembly after the photodetector module, heatsink, and lasermodule of FIG. 9a has been mounted on a header module.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 7a and 7b show the front and side views, respectively, of apreferred semiconductor laser module 100 for generating a laser beam. Inthe preferred embodiment, the laser module includes a laser diode chip102 mounted on a semiconductor chip 104. The laser diode chip has aforward facet 102f and a rear facet 102r for emitting laser light. Thesemiconductor chip has a photodetector diode 106 for detecting the rearfacet emissions of the laser diode in order to maintain a constant laseroutput. The semiconductor chip also has a reverse surge current diode108 for preventing reverse surge currents from damaging the laser diode.

The semiconductor chip is made from a silicon wafer substrate that ismade highly conductive with a deep diffusion of arsenic or antimony. Thephotodetector diode and the reverse surge current diode are formed byimplanting (diffusing) into a silicon wafer to form the diodes at thesubstrate-diffusion junction. In order to simplify fabricationprocedures, both of the diodes are made photosensitive. During ametallization step, wire bond areas areas are positioned at locations109, 110, 112, and 114. Reverse current diode 108 is covered by layer ofmetallization 109 to render the diode nonsensitive to light and to serveas a wire bond area to the anode of the diode. Small metallized area 110is positioned to provide a wire bond area to the anode of thephotodetector diode. Due to the conductivity of the laser chipsubstrate, the cathodes of the photodetector diode and the reverse surgecurrent diode are in common. The common cathode of both diodes has awire bond area at surface metallization 112 or at edge metallization 114of the semiconductor 104 chip as shown in FIG. 7b.

The laser diode chip is bonded to metallization area 112 so that onepole of the laser diode (preferably the anode) is in contact withmetallization area 112, thereby putting the anode of the laser diode incommon with the cathode of the other two diodes. In addition, the otherpole of the laser diode (preferably the cathode) is available for wirebonding at the top surface of the laser diode chip parallel tometallization area 112.

FIGS. 8a and 8b show the side and top views, respectively, of apreferred photodetector module 120 for detecting reflected laser light.In the preferred embodiment, the photodetector module includes a sixsegment photodetector 122 in a photodetector chip 124 which is mountedon a thermally conducting heatsink block 126.

Similar to the semiconductor chip shown in FIGS. 7a and 7b,photodetector chip 124 is made from a silicon wafer that is made highlyconductive with a deep diffusion of arsenic or antimony. The six segmentphotodetector is formed by implanting (diffusing) into the photodetectorchip substrate to form the photodetector diodes at thesubstrate-diffusion junction. During a metallization step, wire bondareas areas are positioned at locations 128a-f for connections to theanodes of the individual photodetectors. Due to the conductivity of thephotodetector chip substrate, the cathodes of the photodetectors are incommon and are accessible for wire bonding at a metallized area 130.Areas 132a-b and 134a-b are also metallized for wire bond to thesubstrate.

The photodetector chip is then attached to heatsink block 126. If theblock is made of electrically conductive materials such as copper, thenan electrical insulator layer 136 needs to be placed between the chipand the block. If the block is made of an electrically nonconductive yetthermally conductive material such as beryllium oxide (BeO), then theelectrically insulating layer is not needed. The photodetector chip 124is then saw cut at locations 131 to divide the chip into three chips124a, 124b, and 124c.

FIG. 9a is a side view of photodetector module 120 after laser module100 has been mounted on the heatsink block 126 and the appropriate wireconnections are made. Semiconductor chip 104 is first die attached to aside surface of the heatsink so that the top surface of thesemiconductor chip is parallel to the side surface of the heatsink. Aswith photodetector chip 124, if the heatsink is made of electricallyconductive materials, then an electrical insulator layer needs to beplaced between the chip and the heatsink. In the preferred embodiment,the laser diode chip 102 is mounted on the semiconductor chip asdescribed with FIGS. 7a and 7b after the semiconductor chip has beenmounted on the heatsink block.

A wire 140 connects the anode of laser diode 102 to the anode of reversesurge current diode 108. A second wire 142 connects the anode of thereverse surge current diode to wire bond area 134b. A third wire 144connects the anode of photosensitive diode 106 to wire bond area 132b.

FIG. 9b shows the electrical characteristics of the device shown in FIG.9a. The anode of laser diode 102 and the cathodes of photodetector diode106 and reverse surge current diode 108 are electrically coupledtogether as described in FIGS. 7a and 7b. The cathode of the laser diodeis electrically coupled to the anode of the reverse surge current diodewith wire 142. In this configuration any accidental negative currentoccurring in the electrical circuit for delivery current to the laserdiode will pass through the reverse surge current diode instead of thelaser diode. As a result, the laser diode is protected from negativesurge current that may occur in an electrical circuit connected to thelaser assembly.

FIGS. 10a and 10b show the top and side views, respectively, of apreferred laser assembly 150 after photodetector module 120, heatsinkblock 126, and laser module 100 has been mounted on a header module 160.The header module includes a head 162 for mounting the heatsink and tenpins 164a-j having an outer end for external connections and an innerend for wire bonding. The heatsink is bonded to the header so that thetop of the photodetector module and the side of the laser module areparallel to the inner end of each pin.

Ten wires 166a-j are then bonded to the modules and the inner ends ofthe pins in order to electrically couple the various elements of thelaser assembly to the ten pins. Wire 166a connects pin 164a with surface114 of the laser module thereby electrically coupling the laser diodeanode, the photodetector diode cathode, and the reverse surge currentdiode cathode with the pin. Wires 166b, 166d, 166f, 166g, 166h and 166jconnect pins 164b, 164d, 164f, 164g, 164h and 164j with the six anodesof the six segment photodetector. Wire 166e connects pin 164e with thecommon cathode of the six segment photodetector. Wire 166c connects pin164c with chip 124a of the photodetector module, thereby electricallycoupling the laser diode cathode and the reverse surge current diodeanode with the pin. Wire 166i connects pin 164i with chip 124c of thephotodetector module, thereby electrically coupling the photodetectordiode anode with the pin.

This preferred embodiment of the laser assembly has the advantage thatthe header and pins do not need to be rotated during the bondingprocess. The dividing of the laser assembly into modules and the singlesurface wire bonding allows the use of standard commercially availabledie attaching and wire bonding machines to produce units in high volume.

While the above provides a full and complete disclosure of the preferredembodiments of the invention, various modifications, alternateconstructions and equivalents may be employed. For example, thephotodetector for detecting the reflected laser beam may have more orless segments than six, the reverse surge current diode may be fabricateas a non-photosensitive diode, and the wire bond blocks may befabricated separately from the photodetector. Accordingly, the abovedescription and illustrations should not be construed as limiting thescope of the invention, which is defined by the appended claims.

What is claimed is:
 1. A semiconductor laser assembly comprising:aheatsink having first and second orthogonal surfaces; a firstsemiconductor photodetector mounted on said first surface of saidheatsink; first and second conductive blocks mounted on said firstsurface of said heatsink, said conductive blocks being electricallyisolated from each other and said first photodetector, each of saidconductive blocks having first and second surfaces parallel to saidfirst and second surfaces of said heatsink, respectively, each of saidfirst and second surfaces of said first and second conductive blocksbeing of sufficient size to act as a wire bonding pad; a semiconductorchip mounted on said second surface of said heatsink, including a secondsemiconductor photodetector and a conductive region having first andsecond surfaces parallel to said first and second surfaces of saidheatsink, respectively, said first surface of said conductive regionbeing of sufficient size to act as a wire bonding pad; a semiconductorlaser coupled to said second surface of said conductive region; a firstwire connected between the semiconductor laser and the second surface ofthe first conductive block; a second wire connected between the secondphotodetector and the second surface of the second conductive block; athird wire connected between the first surface of the conductive regionand a first pin; a fourth wire connected between the first surface ofthe first conductive block and a second pin; and a fifth wire connectedbetween the first surface of the second conductive block and a thirdpin.
 2. A semiconductor laser assembly comprising:a heatsink havingfirst and second orthogonal surfaces; a first semiconductorphotodetector mounted on said first surface of said heatsink; first andsecond conductive blocks mounted on said first surface of said heatsink,said conductive blocks being electrically isolated from each other andsaid first photodetector, each of said conductive blocks having firstand second surfaces parallel to said first and second surfaces of saidheatsink, respectively, each of said first and second surfaces of saidfirst and second conductive blocks being wire bonding pads; asemiconductor chip mounted on said second surface of said heatsink,including a second semiconductor photodetector and a conductive regionhaving first and second surfaces parallel to said first and secondsurfaces of said heatsink, respectively, said first surface of saidconductive region being of sufficient size to act as a wire bonding pad;a semiconductor laser coupled to said second surface of said conductiveregion; and a reverse surge current diode on the semiconductor chipelectrically coupled to the semiconductor laser.
 3. The semiconductorlaser assembly of claim 2 wherein the reverse surge current diode is aphotosensitive diode and covered with metal.
 4. The semiconductor laserassembly of claim 2 further comprising a wire connected between thesemiconductor laser and the reverse surge current diode.
 5. Asemiconductor laser assembly comprising:a heatsink having first andsecond orthogonal surfaces; a first semiconductor photodetector mountedon said first surface of said heatsink; first and second conductiveblocks mounted on said first surface of said heatsink, said conductiveblocks being electrically isolated from each other and said firstphotodetector, each of said conductive blocks having first and secondsurfaces parallel to said first and second surfaces of said heatsink,respectively, each of said first and second surfaces of said first andsecond conductive blocks being wire bonding pads, said conductive blocksbeing formed by mounting a conductive chip to the heatsink and sawcutting the chip; a semiconductor chip mounted on said second surface ofsaid heatsink, including a second semiconductor photodetector and aconductive region having first and second surfaces parallel to saidfirst and second surfaces of said heatsink, respectively, said firstsurface of said conducting region being of sufficient size to act as awire bonding pad; and a semiconductor laser coupled to said secondsurface of said conductive region.
 6. A semiconductor laser assembly forgenerating and detecting a laser beam, the laser assembly comprising:(a)a laser module for generating said laser beam, the laser moduleincluding(i) first and second orthogonal laser module surfaces, (ii) alaser module photodetector (106) on the second surface of the lasermodule having first and second electrodes, (iii) a laser source (102) onthe second surface of the laser module having first and secondelectrodes, (iv) a first laser module wire bond area (110) on the secondsurface electrically coupled to the first electrode of the laser modulephotodetector, (v) a second laser module wire bond area (114) on thefirst surface of the laser module electrically coupled to the secondelectrode of the laser module photodetector, and (vi) a third lasermodule wire bond area (108) on the second surface electrically coupledto the second electrode of the laser source; (b) a photodetector module(120) for detecting said laser beam, the photodetector module havingfirst and second orthogonal photodetector module surfaces and the lasermodule being attached to the photodetector module with the first andsecond laser module surfaces being parallel to the first and secondphotodetector module surfaces, respectively, the photodetector moduleincluding(i) a photodetector module photodetector located on the firstphotodetector module surface having first and second electrodes, (ii) afirst photodetector module wire bond area (132a) located on the firstphotodetector module surface, (iii) a second photodetector module wirebond area (132b) located on the second photodetector module surfaceelectrically coupled to the first photodetector module wire bond area(132a); (iv) a third photodetector module wire bond area (134a) locatedon the first photodetector module surface, and (v) a fourthphotodetector module wire bond area (134b) located on the secondphotodetector module surface electrically coupled to the thirdphotodetector module wire bond area (134a); (c) a first wire (144)connected between the first laser module wire bond area (110) and thesecond photodetector module wire bond area; (d) a second wire connectedbetween the third laser module wire bond area (118) and the fourthphotodetector wire bond area (134b); (e) a header having at least fiveoutput pins for external electrical connections, each pin having a pinwire bond area, each pin wire bond area being parallel to the other pinwire bond areas, the photodetector module being attached to the headerwith the second photodetector module surface and the second laser modulesurface being parallel to the pin wire bond areas; (f) a third wire(166a) connected between the second laser module wire bond area and oneof the pin wire bond areas; (g) a fourth wire (166c) connected betweenthe first photodetector module wire bond area and one of the pin wirebond areas; and (h) a fifth wire (166i) connected between the thirdphotodetector module wire bond area and one of the pin wire bond areas.7. The laser assembly of claim 6 wherein the laser source comprises asemiconductor laser diode.
 8. The laser assembly of claim 7 wherein thelaser module further comprises a reverse surge current diode forpreventing reverse surge voltage, the reverse surge current diode havinga first electrode and a second electrode, the first electrode of thereverse surge current diode being electrically coupled to a secondelectrode of the laser source and the second electrode of the reversesurge current diode being electrically coupled to the first electrode ofthe laser source.
 9. The laser assembly of claim 8 wherein the reversesurge current diode is a photosensitive diode located on the secondsurface of the laser module, the reverse surge current diode beingcoated with metal to prevent the diode from being photosensitive.